This invention relates to an optical disc base plate, particularly to an optical disc base plate read out by light and used for digital audio discs, optical video discs, memory discs, magnetic optical discs, etc.
Optical discs are formed, for example, by placing oppositely two optical disc base plates comprising a transparent substrate such as a glass plate, a transparent plastic plate, or the like, a primer layer formed on the substrate and having groove-like and hole-like depressions and protuberances for servo tracks, and a metal-made recording film formed on the primer layer using such a metal as Bi, In, Te, As, Pb, Sn or the like, so as to face the recording films each other using a spacer with a predetermined distance, and bonding the spacer and the transparent substrates with an adhesive. Then, the recording film is irradiated with a light beam and the irradiated portion of the metal-made recording film is removed to give information units.
More concretely, such optical discs have been obtained by inserting an ultraviolet-curable resin composition between a stamper made of a metal such as nickel and having predetermined depressions and protuberances, and a transparent substrate, curing the resin composition with ultraviolet light to form a primer layer, taking off the metal-made stamper, forming a metal-made recording film on the primer layer having depressions and protuberances, placing two thus formed optical disc base plates (comprising the transparent substrate, the primer layer and the recording film) oppositely so as to face the metal-made recording films each other using spacers with a predetermined distance, and bonding the spacers to the optical disc base plates.
On the other hand, it has also been known that optical discs can be produced by forming a transparent substrate having a function of primer layer and predetermined depressions and protuberances by using a thick primer layer made from a ultraviolet curable resin composition and omitting a conventionally used transparent substrate, forming a metal-made recording film on the surface of the transparent substrate to give an optical disc base plate, placing two optical disc base plates oppositely so as to face the metal-made recording films each other by using spacers with a predetermined distance and bonding the spacers to the optical disc base plates.
The ultraviolet-curable resin composition used for the primer layer mentioned above should satisfy the following conditions:
(a) The cured product is optically transparent.
(b) The resin adheres well to the transparent substrate made from glass, a transparent plastic, or the like.
(c) The resin is easily peeled from the stamper.
(d) The cured product does not generate a substance corrosive to the metal-made recording film.
(e) Recorded signals can be read out accurately without deforming the metal-made recording film due to moisture absorption even if used for a long period of time.
(f) Thermal stability of grooves with depressions and protuberances on the metal-made recording film obtained by vacuum deposition is excellent without causing changes in grooves' height and shape.
(g) To have mechanical strength enough to withstand a high-speed revolution of 500 rpm or more is necessary.
But conventional ultraviolet-curable resin compositions were not able to satisfy the above-mentioned conditions (a) to (g) at the same time.
A typical example is shown in Japanese Patent Unexamined Publication No. 127946/81.